Influence of Okhotsk Sea blocking on summer precipitation over South Korea

Abstract

This study examined the influence of atmospheric blocking on the variability of precipitation over South Korea during summer (June–July–August) by defining the blocking frequency over the Okhotsk Sea (Okhotsk Sea blocking frequency; OK_BF). According to composite analysis for the years of high precipitation over South Korea, blocking occurs more frequently over the Okhotsk Sea (140°E–160°E). Partial correlation and regression analyses were conducted to separate the contribution of OK_BF to precipitation variability from that of the low‐level meridional wind (LLMW) because LLMW over the region is another important aspect of summer precipitation in South Korea. The barotropic structures of positive geopotential height anomalies over the Okhotsk Sea associated with increasing OK_BF can induce negative temperature anomalies over South Korea due mainly to the equatorward advection of cold air masses from higher latitudes. The enhanced meridional temperature gradient can cause increases in baroclinic instability and zonal wind vertical shear according to the thermal wind balance. This instability can induce anomalous cyclonic circulations over South Korea, resulting in positive precipitation anomalies. The partial correlation coefficients (R2 = 0.35–0.40) between the OK_BF and precipitation indices, including mean precipitation, extreme precipitation intensity, wet days, and consecutive wet days, were all statistically significant at the 95% confidence level. Overall, the effects of the increasing OK_BF on both precipitation and potential evapotranspiration can intensify the surface water budget in South Korea.

Characteristics of precipitation in East Asia associated with Okhotsk Sea blocking in June and July

“…The occurrence of OKB events during the boreal summer has an impact on mean summer precipitation in East Asia (e.g., Park & Ahn, 2014; Song & Ahn, 2022). There are remarkable differences in spatial structures of the climatological rainband between early and late summer, which are related to the northward march of the WNPSH ridge during the summer.…”

“…This study specifically focuses on blocking the ridge in the Okhotsk Sea (140° E–160° E). OKB is known to have the most significant influence on precipitation in East Asia (Park & Ahn, 2014; Song & Ahn, 2022). Similar to the method described by Song and Ahn (2022), an OKB event is present if the blocked longitudes cover more than 40% (i.e., four grid points) within the target area (i.e., nine grid points; 140° E–160° E), and if the duration of the event is 5 days.…”

“…OKB is known to have the most significant influence on precipitation in East Asia (Park & Ahn, 2014; Song & Ahn, 2022). Similar to the method described by Song and Ahn (2022), an OKB event is present if the blocked longitudes cover more than 40% (i.e., four grid points) within the target area (i.e., nine grid points; 140° E–160° E), and if the duration of the event is 5 days. A total of 39 OKB events are identified during the summers of 1979–2022 (Table 1).…”

“…Most studies of the relationship between climate and blocking over the Northern Hemisphere focus on winter or spring when blocking is most frequent (e.g., Cheung et al, 2013; Kim & Ahn, 2022; Wiedenmann et al, 2002). However, the extreme weather events associated with blocking, such as heat waves and heavy rain, have recently been reported in the Northern Hemisphere during summertime (e.g., Chen et al, 2021; Choi et al, 2022; Li et al, 2020;Park & Ahn, 2014; Song & Ahn, 2022). The blocking phenomenon is directly related to the dipole precipitation anomaly pattern, which is characterized by drier conditions in the blocked regions and wetter conditions south of the blocked regions (e.g., Fernandes & Rodrigues, 2018; Rodrigues & Woollings, 2017; Sousa et al, 2016, 2017).…”

“…The occurrence of a blocking anticyclone in the Sea of Okhotsk (hereafter, Okhotsk Sea blocking or OKB) significantly affects the summer climate of East Asia (Park & Ahn, 2014; Song & Ahn, 2022). Studies have shown that frequent OKB has a negative (positive) impact on temperature (precipitation) in East Asia.…”